This invention relates to large commercial building air conditioning systems, and is particularly directed to an improvement in the storage of thermal energy for leveling of peak electrical demands required by air conditioning systems. More specifically, the invention is directed to techniques for storing thermal energy by freezing water to ice during off-peak hours, and thereafter, during peak electrical demand hours, by melting the ice for recovering the thermal energy to chill a heat exchange liquid that flows to cooling coils, e.g., of room air units or the like.
With an ice thermal storage tank, it is possible to use the air conditioner chiller at night or weekends, i.e., when the building is closed, to freeze the ice in the tank and store thermal energy. Thereafter, during the next business day the ice can be melted to recover cooling power and cool the office space. This permits a smaller chiller to be used for an equivalent cooling power, which would be run steadily, but at a level well below peak. With the ice thermal storage system, it is possible to shave peak usage during the normal business day when there is an excessive electrical demand.
In a typical ice thermal storage system, there are a number of ice storage tanks provided, each of which comprises a tank enclosure which is filled with water, and a coiled brine tube i.e. heat exchange tube, which can be plastic, copper, or other suitable material, through which a cold brine flows during times that ice is to be produced, and through which warm brine flows during times that the ice is to be used for cooling the building. As used in this application, and as is understood in the art, the term "brine" refers to a heat exchange liquid which will not freeze at the water freezing point, such as ethylene glycol or an aqueous solution of ethylene glycol.
While the freezing point of water is 32.degree. F., it has been found that the water does not freeze spontaneously at that temperature. Rather, it is usually necessary to supercool the water down to about 26.degree. F. to commence ice formation. It is believed that this is necessary because the ice storage tank lacks suitable nucleation points. Attempts to provide the brine tube with a rough or irregular surface have not produced good results.
The requirement to supercool the water below 32.degree. F. is highly disadvantageous. Although the water must be superchilled to 26.degree. F. to freeze, when the ice is later melted to recover the stored cooling power, the ice melts at 32.degree. F., not at 26.degree. F. This temperature difference represents a hysteresis loss, i.e., wasted energy. Further, the air conditioning chillers are designed for a brine temperature of about 35.degree. F. The conventional ice storage system imposes an excessive load on the chiller if it is necessary to cool the brine down to 26.degree. F. to form ice in the tank. There would be a significant reduction in strain on the chiller if it were possible to form ice at a higher temperature, i.e., in the range between 30.degree. and 32.degree. F.